This invention relates generally to fuel cells and more particularly to methods and fuel cell systems for regulating a supply of reformate to a fuel cell.
Fuel cells electrochemically convert fuels and oxidants to electricity and they can be categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid, or solid polymer) used to accommodate ion transfer during operation. Moreover, fuel cell assemblies can be employed in many (e.g., automotive to aerospace to industrial to residential) environments, for multiple applications.
A Proton Exchange Membrane (hereinafter xe2x80x9cPEMxe2x80x9d) fuel cell converts the chemical energy of fuels such as hydrogen and oxidants such as air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (i.e., H+ions) from the xe2x80x9canodexe2x80x9d side of a fuel cell to the xe2x80x9ccathodexe2x80x9d side of the fuel cell while preventing passage therethrough of reactant fluids (e.g., hydrogen and air/oxygen gases).
Usually, an individual PEM-type fuel cell assembly or stack has multiple, generally transversely extending layers assembled in a longitudinal direction. In the typical fuel cell assembly or stack, all layers which extend to the periphery of the fuel cells have holes therethrough for alignment and formation of fluid manifolds that generally service fluids for the stack. As is known in the art, some of the fluid manifolds distribute fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) to, and remove unused fuel and oxidant as well as product water from, fluid flow plates having fluid flow channels.
As is known in the art, the PEM can work more effectively if it is wet. Conversely, once any area of the PEM dries out, the fuel cell does not generate any product water in that area because the electrochemical reaction there stops. Undesirably, this drying out can progressively march across the PEM until the fuel cell fails completely. To avoid this, the fuel and oxidant fed to each fuel cell are usually humidified.
In addition to it being undesirable for a fuel cell to dry out, it is also undesirable for fuel cells to flood. In residential use of fuel cells, less electrical demand at off-peak times can yield lower flow rates, of reactant fluids which may result in the accumulation of water in the fluid flow channels. One method of removing accumulated water from the fluid flow channels is to temporarily increase the flow rate of reactant fluids (e.g., reactant fuel) and air to sweep the water away and then return to an appropriate flow rate. This results in energy (e.g., reactant fuel) being wasted in order to remove water from the flow channels.
Thus, there is a need for an efficient method and fuel cell system which inhibits the flooding of fuel cells particularly in periods of low electrical demand.
The present invention provides, in a first aspect, a method for regulating a supply of reformate to a fuel cell in which the method includes receiving exhaust oxidant from the fuel cell, combining the exhaust oxidant and fuel to form reformate, and providing the reformate to the fuel cell.
The present invention provides, in a second aspect, a fuel cell system which includes a fuel cell, a reformer for forming reformate for use in the fuel cell, and wherein exhaust oxidant from the fuel cell and fuel are combined to form the reformate.